fix problem with eigen library

Tracking/Acts/FaserActsKalmanFilter/FaserActsKalmanFilter/LinearFit.h

 #ifndef FASERACTSKALMANFILTER_LINEARFIT_H
 #define FASERACTSKALMANFILTER_LINEARFIT_H
 
-#include "eigen3/Eigen/Dense"
+#include "Acts/Definitions/Algebra.hpp"
 
 namespace LinearFit {
 
-using Vector2 = Eigen::Matrix<double, 2, 1>;
+using Vector2 = Acts::Vector2;
 
 std::pair <Vector2, Vector2 > linearFit(const std::vector<Vector2> &points) {
   size_t nPoints = points.size();